Practice question

Tonicity governs cell volume by defining relative effective osmolarity of extracellular fluid compared with cytoplasm, usually near 300 milliosmoles. Hypertonic environment contains higher concentration of non-penetrating solutes like sodium chloride, mannitol or sucrose, leading to lower water potential described by psi equals psi solute plus psi pressure. Since water diffuses toward lower water potential via aquaporins and lipid bilayer, net efflux occurs from cell to surroundings. Loss of water concentrates intracellular proteins and solutes, decreases cell volume, increases ionic strength and may perturb enzyme activity and signaling cascades. Animal cells without rigid wall shrink, developing crenated or spiky morphology as cortical cytoskeleton collapses. In plants large central vacuole loses water, turgor pressure drops, plasma membrane retracts from cell wall during plasmolysis, causing wilting and growth arrest. Regulatory volume increase mechanisms involving NKCC, NHE and Cl transport may later import osmolytes to recover, but immediate physical response to hypertonic challenge is passive water loss leading to shrinkage and crenation.